There is a growing interesting in the development of new passive optic polymer waveguide materials for telecommunication applications such as a thermo-optic switching, optical wavelength filters, beam splitters, optical connectors and arrayed waveguide gratings (AWG). Polymers, because of their excellent low-temperature processability and their ability to be chemically modified or blended with other polymers, are ideal candidates as waveguide materials, since the optical properties can be tailored to requirements. Polymer waveguides also offer the potential to be incorporated into highly complex integrated devices and optical interconnects on a planar substrate.
The requirements for the ideal passive optical polymer material are:                Low optic loss at 1.3˜1.55 μm.        Low birefringence, Δn<5×10−5.        Adjustable refractive index.        Crosslinkable (photo- or thermal-). Good substrate adhesion.        High mechanical strength.        High Tg (>120° C.).        Good processing properties (coating, etching, dicing, etc.)        High durability when incorporated into a device (e.g. high Tg, low water up-take, high chemical and environmental resistance.        
Many attempts have been made to produce polymers that meet the above criteria. For example, several polymers have been prepared in which fluorine or deuterium has been used to replace hydrogen in the molecular structure. Polymers prepared with these substituents have been shown to reduce optic losses. However, when these materials have been tested in optic waveguides applications, only a few have shown satisfactory performance (these materials are summarized in Table 1). In terms of waveguide applications, the polymers based upon the fluorinated polyethers (FPAE and FPEK) are considered to be the best candidates, since they offer materials with: low optic loss, low birefringence, and good mechanical properties. However, based upon the “ideal” criteria listed above, it should be noted that even these materials fail to meet the criteria for optic loss and birefringence.
TABLE 1Polymers used for planar passive optic waveguidesloss,Tg1.55 mmPolymer(° C.)(dB/cm)NΔnnotesPFPs-PGMA82–970.42 1.46–1.4754 × 10−4RIT, SwedenF-polyacrylate100–1500.61.3~1.5AlliedSignalPFCB120–3500.21.46–1.54Clemson U.DowFPAE167(240)*0.21.495–1.530  7.8 × 10−3KoreaFPEK149(202)*<0.5  1.51  1.4–4.6 × 10−3Monash U.Koreapolycyanurate~2500.6~1.51GermanyCl-Polyimide0.41.51–1.57  1.0 × 10−2KoreaF-Polyimide0.51.52–1.55  0.57–1.58 × 10−2Korea*crosslinked sample
In terms of chemical structure, one way to achieve a low optical loss material is to replace the hydrogen atoms in a polymer structure with fluorine atoms. Consequently the fluorinated polyethers (FPAE and FPEK), listed in Table 1, would be expected to have the lowest optical losses because they have higher fluorine content. Meanwhile, in order to obtain materials with variable refractive index the chemical structure of the polymers can be modified. This can be achieved by the incorporation of aliphatic groups, which will reduce the refractive index, or alternatively using aromatic groups, which increase the polymers higher refractive index. In addition a factor that affects a polymeric materials the birefringence is the chain orientation. Reducing the orientation in a polymer yields materials with low birefringence. The following represent ways of reducing the orientation to achieve a low birefringence material:                Highly flexible polymer chain.        Lower glass transition temperature polymers. However, it should be noted that this conflicts with the reliability of the device. The lowest Tg recommended for a durable reliable device is 120° C.        Low processing temperature (crosslinking, annealing, etc.)        
Kim et al in Macromolecules (2001), 34:7817–7821 describe a process for preparing fluorinated poly(arylene ether sulfide) for polymeric optical waveguide devices employing a high temperature (120° C.) to ensure complete dehydration.
U.S. Pat. No. 6,136,929 (Han et al) discloses a method for making polyarylene ethers employing K2CO3 at 80° C. for 24 hours.
Japanese patent, JP2002194082 (Lee et al) discloses the preparation of fluorinated poly(arylene ether sulfide) and poly(arylene ether sulfone) for polymeric optical waveguide devices using azeotropic distillation at 120° C. for the removal of H2O.
One of the drawbacks of the published techniques for the production of fluorinated polyethers is the high tendency of the side reaction on the ortho-position of bis(pentafluoro phenyl) compounds, which leads to branching structures and even crosslinked microgels in the products.
There are 10 fluorines in the bis(pentafluorophenyl) compounds, and both para- and ortho-fluorines are reactive in the polycondensation reaction. Any reaction of ortho-fluorines will cause undesirable branching and even crosslinking structures, which is detrimental for the optical applications. Therefore, for preparing useful linear polymers, the selectivity of the reaction to the para-fluorines should be high.
Unfortunately, for the monomers with electron withdrawing group such as ketone, sulfone or oxadiazole as the X group (see Scheme 1), the selectivity is relative poor, and large amount branching structures, even crosslinked microgels were proved to form in the products by using the above mentioned techniques if the polymers with high molecular weight were prepared.
In the present invention, the polycondensation reaction was initially modified by the addition of a dehydrating thimble filled with molecular sieves or calcium hydride to dehydrate the condensed solvent from refluxing, which enables the preparation of linear polymers from a wide range of monomers with different linkage group X as listed in Scheme 1.
The polycondensation reaction has been further modified by using a CaH2 mediated technique. This modified reaction is especially good for the preparation of the fluorinated aromatic polyethers from activated bis(pentafluorophenyl) compounds with electron withdrawing group (such as ketone, sulfone or oxadiazole) as the linkage unit X.
We have found that this novel process offers a wide range of advantages over existing processes. These include the following: mild reaction condition, less side reaction, the obtained product is free of any gel particles, white in colour, and the reaction is simple, fast and has a high degree of reproducibility and is easy to control and the reaction is applicable to many starting materials as described in Scheme 1 (see below).
Another drawback of the published techniques relates to the means of achieving the crosslinkability of the polymers. Because crosslinkable polymers have to be used in waveguide fabrication, crosslinking groups have to be introduced into polymers at the chain end or as side pendant groups. Based upon published information, only phenyl ethynyl or ethynyl groups have been suggested as the means of introducing crosslinking ability to the polymers. The reactions associated with these techniques involve a two-step process. First the polymer has to be prepared and purified, and then the purified polymer can be reacted with 4-phenyl ethynyl phenol (PEP) or with 3-ethynylphenol (EP) to yield the crosslinkable polymer with the crosslinker at the chain end.
There are several disadvantages associated with this technique:                Of the polymers prepared using this approach (only two) one is believed to have high impurity content making its use impractical for normal applications.        The crosslinking group is only attached to the chain end, thus its content in polymer is limited to a very low level.        PEP and EP are not commercial available and are difficult to prepare.        PEP and EP are not fluorinated compounds and the resultant polymers possess low fluorine content which give higher optical loss materials.        The polymers have to be cured at high temperatures (350° C. for PEP polymer and 250° C. for EP polymer), which results in the formation of cured materials with high birefringence. In addition the curing at high temperature causes increases the chances of side reactions such as oxidation. These side reactions contribute to larger optical losses.        
The process of this invention provided a simple approach for introducing crosslinkable fluorostyrene moieties as shown in Scheme 1 into the polymers with an adjustable concentration by a one-pot reaction. Comparing to the published techniques, this invention possesses following advantages for the process and materials:                (a) The product is obtained as a pure white polymer with a low PDI.        (b) The product is free of any crosslinked structures        (c) Polymers with higher molecular weight are possible (Mw˜50,000 Da)        (d) The process is suitable for introducing FSt into polymer for crosslinking.        (e) The contents of FSt in the polymer are variable and can be as designed.        (f) The product is photo- and thermally-crosslinkable.        (g) Low or high curing temperatures could be employed (ambient temperature to 250° C.).        (h) The product has an idealized Tg (e.g., 140° C. before curing, 170° C. after curing for FPEK-FSt, and 163° C. before curing, 191° C. after curing for FPESO-FSt)        (i) The product has a low birefringence.        (j) A range of polymers can be prepared covering a wide range of refractive index.        (k) The product produces uniform films, with excellent reproducibility in their optical properties (see below).        
Most of the published synthetic techniques involve using K2CO3 or other alkali carbonate to neutralize HF that is produced in the reaction, and thus H2O is produced from the reaction. It has to be removed from the solution in order to complete the reaction and to eliminate the side reactions such as hydrolysis caused by H2O. Azeotropic distillation are a common used technique for this purpose in the preparation of fluorinated poly(arylene ethers). However, this technique can not sufficiently remove H2O from the reaction and thus severe reaction conditions (high temperature, long reaction time) have to be employed in order to yield a high conversion for high molecular weight polymers. This severe condition causes side reactions (hydrolysis, cleavage, cyclization, oxidation, etc.), and lead to polymers with lower MW, high PDI and colour. On the other hand, for the azeotropic distillation, non-polar solvent, benzene or toluene has to be introduce into the reaction, which will reduce the selectivity of the reaction on the para-position of bis(pentafluorophenyl) compounds, and results in higher content of branching and even crosslinking structures.